Topsy-turvy Stress built up within the outer layer of the Earth helps snap our planet back into position whenever it strays from its rotational axis, according to a US study.

A report in the journal Nature explains the phenomenon known as "oscillatory true polar wander", which has occurred at least five times in the last 2 billion years.

Previous research looking at the magnetic signatures found in ancient rocks has identified these events, also known as Earth 'tipping'. They are different to flips in the magnetic poles - caused by the rotation of fluids in the outer core - or the precession of the Earth's rotational axis.

"We knew the tipping was caused by the convection of materials of different compositions, densities and temperatures in the Earth's mantle," says Dr Jessica Creveling of Harvard University in Massachusetts, and lead author of the study.

She says over time, the distribution of mass within the Earth changes, causing the direction of its rotational axis to move.

"We wanted to know why the Earth tips back again."

To answer this question, Creveling and colleagues used computer simulations of the Earth.

"Our simulation shows how the interior of the Earth moves and creeps through time, with plumes of mantle material welling up and down, moving mass," says Creveling.

This movement creates stresses in the Earth's tectonic plates and lithosphere.

"And just like a rubber band that's stretched, those plates want to snap back to their original size."

According to Creveling, the Earth's imperfect spherical shape - longer around the equator than around the poles - also helps right the planet back to its original position.

"We found this equatorial bulge acts as a stabiliser righting the planet again, after it tips."

Favours supercontinents

The researchers also found that tipping events are more likely to occur when there is one large landmass and few tectonic plates.

The last time this occurred was during the time of Pangaea, between 250 and 150 million years ago, when the planet tipped 22 degrees, followed by another smaller tip of eight degrees between 150 and 100 million years ago.

Creveling says such an event would be unlikely to occur today.

"The fact that we have lots of tectonic plates, that is, the Earth's surface is broken up into many pieces ... means that conditions are not favourable for a tip over event right now."

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